The present invention concerns data transfer over a network and pertains particularly to a link monitor state machine used in a physical media access sublayer of 100BASE-TX technology.
The IEEE 802.3 committee has defined a standard technology for 100 megabits per second networking over category five (CAT-5) Unshielded Twisted Pair (UTP) cabling. This technology is known as 100BASE-TX and is defined in Clauses 24 and 25 of the specification IEEE 802.3u-1995.
Within the 100BASE-TX technology physical sublayer (PHY), as defined in Clauses 24 and 25 of the specification IEEE 802.3u-1995, there is a Physical Coding Sublayer (PCS), a Physical Media Access (PMA) sublayer, and a Physical Media Dependent (PMD) sublayer. The PCS defines how data is encoded and decoded, how the Carrier Sense (CS) and Collision Detection (CD) functions work, and the interface between higher and lower layers in the protocol specification. The PMA defines the mapping of code bits, generation of a control signal (link_status) which indicates the availability of the PMD, generation of control signals to the PCS that indicate Carrier Sense, Collision Detection and Physical Layer Errors, and clock recovery. The PMD defines the signaling method and the various physical parameters that are necessary to address the link's physical requirements. Within the PMA, as defined in the specification IEEE 802.3u-1995, there is a state machine which is described as the LINK MONITOR state machine. It is described in section 24.3.4.4 and by a diagram depicted in FIGS. 24-15. This state machine is intended to provide a mechanism which determines whether the underlying physical layer is providing reliable data.
The LINK MONITOR state machine described in section 24.3.4.4 and by a diagram depicted in FIGS. 24-15 of the specification IEEE 802.3u-1995 assumes that if a link has been connected and the signal energy is sufficient to cause an indication of "signal_status ON" for more than 330 micro-seconds, then the link must be reliable. However, a link which exceeds the signal amplitude requirements to generate a "signal_status ON" may in fact have a Bit Error Rate (BER) that is substantially above the acceptable level as defined by requirements set out in the specification IEEE 802.3u-1995. A link which has a Bit Error Rate (BER) that is substantially above the acceptable level is unreliable.
When using the LINK MONITOR state machine described in section 24.3.4.4 of the specification IEEE 802.3u-1995, once a link has been enabled to operate, it will go to a state known as "LINK UP,` and stay there until either the signal_status goes to OFF or a control signal is asserted to bring the link down. The BER of the link has no direct impact on link_status.
The 100BASE-TX technology uses a protocol known as Carrier-Sense, Multiple Access with Collision Detection (CSMA-CD). The CSMA-CD protocol depends on the reliable ability of each node to detect when another node in the network is transmitting (Carrier Sense) and when more than one node in the network is transmitting (Collision Detection). If bit errors occur during a packet, a re-transmission may occur which reduces network performance. However, if a protocol error occurs, the impact upon network performance degrades substantially.
The 100BASE-TX technology uses a continuous signaling mechanism to communicate across the link. This means that the lack of "Carrier" is actually a stream of bits known as "IDLE" symbols. If a bit error occurs in the reception of the IDLE symbols, then "Carrier Sense" or "Collision" may be detected. Therefore, this System absolutely depends on a reliable BER to operate properly. An unacceptable BER on any link in the network will result in substantial disruption and degradation of network performance.